1. What is A-a Gradient?

The Alveolar-arterial (A-a) gradient measures the difference between alveolar oxygen partial pressure and arterial oxygen partial pressure. It helps differentiate causes of hypoxemia and assess gas exchange efficiency in the lungs.

2. How Does the Calculator Work?

The calculator uses the A-a gradient equation:

Where:

• \( FiO_2 \) — Fraction of inspired oxygen (0.21 for room air)
• \( P_{atm} \) — Atmospheric pressure (760 mmHg at sea level)
• \( P_{H_2O} \) — Water vapor pressure (47 mmHg at 37°C)
• \( PaCO_2 \) — Arterial carbon dioxide partial pressure
• \( PaO_2 \) — Arterial oxygen partial pressure

Explanation: The equation calculates the alveolar oxygen tension and subtracts the measured arterial oxygen tension to determine the gradient.

3. Clinical Significance

Details: Normal A-a gradient is <10-15 mmHg in young adults and increases with age. Elevated gradient suggests ventilation-perfusion mismatch, diffusion defects, or right-to-left shunts.

4. Using the Calculator

Tips: Enter FiO2 as fraction (0.21 for room air, 1.0 for 100% oxygen), use standard values for Patm (760 mmHg) and PH2O (47 mmHg) unless at altitude or different conditions.

5. Frequently Asked Questions (FAQ)

Q1: What is a normal A-a gradient?
A: Normal is <10-15 mmHg in young adults on room air. It increases by approximately 1 mmHg per decade of life.

Q2: When is A-a gradient elevated?
A: Elevated in V/Q mismatch, diffusion abnormalities, right-to-left shunts, and in elderly patients.

Q3: How does altitude affect A-a gradient?
A: At altitude, atmospheric pressure decreases, requiring adjustment of Patm value in the calculation.

Q4: What is the respiratory quotient (0.8)?
A: 0.8 represents the average respiratory quotient (CO2 production/O2 consumption) in steady state conditions.

Q5: Can A-a gradient be normal in lung disease?
A: Yes, in pure hypoventilation without parenchymal lung disease, A-a gradient remains normal despite hypoxemia.